Conventionally, a derailleur for a bicycle comprises a base member, a movable member having a chain guide, and a support means composed mainly of a pair of linkage members for supporting the movable member movably axially of a multistage sprocket assembly, so that an operating means mainly comprising a control cable is operated to move the movable member through the support means and the driving chain guided by the chain guide is shifted from one to another sprocket at the multistage sprocket assembly for changing the bicycle speed.
The derailleur constructed as foregoing and using a positioning means comprising a plurality of engaging portions and an engaging member engageable with one of the engaging portions has been well-known, with the engaging portions being provided at one of two members movable relative to each other during the movement of the movable member and the engaging member being provided at the other to thereby set the speed change position of the movable member, in turn the chain guide, and maintain the set position.
In a case of using such positioning means, the chain guide has been set of an amount of movement and has to move in every preset amount. Hence, for the multistage sprocket assembly whose each sprocket is spaced at an interval to mate the amount of movement of chain guide, the chain guide can stop at the position corresponding to the widthwise center line of each sprocket, which is not problematical. However, for the multistage sprocket assembly not constructed as the above, the chain guide during the speed change cannot be stopped at the proper position corresponding to the widthwise center line of each sprocket.
Therefore, there has been the problem in that the speed change capability deteriorates, or the driving chain, even when shifted, gets out of the proper position to generate sounds due to contact with the sprocket. Hence, in order to solve the above problem, various derailleurs have contentionally been required which can move the chain guide corresponding properly to the interval between each sprocket. On the other hand, in some cases that the positioning means is provided not at the derailleur but at the control cable, or a display means instead of the positioning means is provided at the control cable, the above problem remains unsolved.